feat(lib): use new challenge creation flow

Previously Anubis constructed challenge strings from request metadata.
This was a good idea in spirit, but has turned out to be a very bad idea
in practice. This new flow reuses the Store facility to dynamically
create challenge values with completely random data.

This is a fairly big rewrite of how Anubis processes challenges. Right
now it defaults to using the in-memory storage backend, but on-disk
(boltdb) and valkey-based adaptors will come soon.

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/challenge/interface.go

@@ -0,0 +1,68 @@
+package challenge
+
+import (
+	"log/slog"
+	"net/http"
+	"sort"
+	"sync"
+
+	"github.com/TecharoHQ/anubis/lib/policy"
+	"github.com/TecharoHQ/anubis/lib/policy/config"
+	"github.com/TecharoHQ/anubis/lib/store"
+	"github.com/a-h/templ"
+)
+
+var (
+	registry map[string]Impl = map[string]Impl{}
+	regLock  sync.RWMutex
+)
+
+func Register(name string, impl Impl) {
+	regLock.Lock()
+	defer regLock.Unlock()
+
+	registry[name] = impl
+}
+
+func Get(name string) (Impl, bool) {
+	regLock.RLock()
+	defer regLock.RUnlock()
+	result, ok := registry[name]
+	return result, ok
+}
+
+func Methods() []string {
+	regLock.RLock()
+	defer regLock.RUnlock()
+	var result []string
+	for method := range registry {
+		result = append(result, method)
+	}
+	sort.Strings(result)
+	return result
+}
+
+type IssueInput struct {
+	Impressum *config.Impressum
+	Rule      *policy.Bot
+	Challenge *Challenge
+	OGTags    map[string]string
+	Store     store.Interface
+}
+
+type ValidateInput struct {
+	Rule      *policy.Bot
+	Challenge *Challenge
+	Store     store.Interface
+}
+
+type Impl interface {
+	// Setup registers any additional routes with the Impl for assets or API routes.
+	Setup(mux *http.ServeMux)
+
+	// Issue a new challenge to the user, called by the Anubis.
+	Issue(r *http.Request, lg *slog.Logger, in *IssueInput) (templ.Component, error)
+
+	// Validate a challenge, making sure that it passes muster.
+	Validate(r *http.Request, lg *slog.Logger, in *ValidateInput) error
+}